Force-sensing elements are widely used in scanning force microscopy. Rocking beam techniques have been applied in some of these systems. Rocking beam sensors are potentially advantageous because, in principle, they can be mechanically simple, rugged, highly sensitive, and relatively inexpensive to manufacture. In particular, these sensors offer high performance because they can be stabilized by a force-feedback servo loop. A further advantage of rocking beam sensors is that they can be used for a variety of microforce-sensing applications, including the measurement of tilt or acceleration. However, previous attempts to provide a rocking beam sensor have not been entirely satisfactory. Practitioners have found that pivoting mechanisms that offer sufficient sensitivity in the rocking direction generally tend to be fragile and too easily damaged during routine handling. At least some sensors have been found to be difficult to fabricate. The rocker elements of at least some of these sensors have proven difficult to remove and replace. (Rapid replacement is important in scanning force microscopy, for example. This is because the finely pointed probe tips, which are permanently mounted on the rocker elements, break or wear out relatively quickly.) Moreover, practitioners have encountered difficulties in stabilizing the rocker elements against undesired modes of mechanical motion.
For example, a rocking beam sensor fabricated from a thin sheet of beryllium-copper alloy is described in S. A. Joyce et al., "A new force sensor incorporating force-feedback control for interfacial force microscopy," Rev. Sci. Instrum. 62 (1991) 710-715. This sensor is a differential capacitance displacement sensor in which a common capacitor plate is suspended by torsion bars. The common capacitor plate and the torsion bars are integral with the beryllium-copper alloy sheet. Practitioners using this sensor have found that planarity of the rocker, which is essential to proper operation, is difficult to maintain because the beryllium-copper sheet tends to bend. Practitioners have also found that sensitivity is limited by stiffness of the torsion bars in the rocking direction, and that unwanted degrees of mechanical freedom make a substantial contribution to measurement noise.
An alternative rocking beam sensor is described in G. L. Miller et al., "A rocking beam electrostatic balance for the measurement of small forces," Rev. Sci. Instrum. 62 (1991) 705-709. In this sensor, which is also a differential capacitance displacement sensor, the rocker (which comprises the common capacitor plate) is a silicon beam which pivots on a carbon or tungsten fiber. The fragility of the fiber is an undesirable feature of this sensor. Damage to the fiber introduces undesired degrees of mechanical freedom, causing the signal output to be contaminated with noise. Moreover, this sensor relies on the weight of the rocker to hold it together, and therefore cannot be operated in an inverted orientation.